1. Field of the Invention
The present invention relates to an improved electron beam collector and, more particularly, to a collector having an ion expeller to repel ion build up within the collector and promote efficient electron dispersal.
2. Description of Related Art
Many electronic devices employ a travelling stream of charged particles, such as electrons, formed into a beam as an essential function in the device's operation. In a linear beam device, an electron beam originating from an electron gun is caused to propagate through a tunnel, or drift tube, generally containing an RF interaction structure. Within the interaction structure, the beam must be focused by magnetic or electrostatic fields in order for it to be effectively transported through the interaction structure without energy loss. In the interaction structure, kinetic energy is transferred from the moving electrons of the beam to an electromagnetic wave that is propagating through the interaction region at approximately the same velocity as the moving electrons. The electrons give up energy to the electromagnetic wave through an exchange process characterized as electronic interaction, which is evident by a reduced velocity of the electron beam from the interaction region. These "spent" electrons pass out of the interaction region where they are incident upon and collected by a final element, termed the collector. The collector collects and returns the incident electrons to the voltage source. Much of the remaining energy in the charged particles is released in the form of heat when the particles strike a stationary element, such as the walls of the collector.
After the beam enters the collector, the absence of magnetic focusing causes the individual electrons to disperse due to space charge. Since the electrons have like charges, they naturally repel from each other. The dispersed electrons evenly strike the internal walls of the collector. Typically, heat generated by the impacting electrons is conducted through the collector walls to an external coolant jacket which surrounds the collector.
Collector operation can be significantly degraded if the beam fails to disperse evenly within the collector. Electron impact within the collector often produces positive ions which can build up within the collector and cancel the space charge. The absence of space charge within the collector can cause the electrons to remain focused in the beam. If the concentrated beam were to strike a single spot within the collector, rather than being dispersed, the beam could quickly overstress or damage the collector. High power beams which operate at near relativistic velocities tend to develop a self induced magnetic field which also contributes to keeping the beam focused after entering the collector.
Thus, it would be desirable to provide an electron beam collector capable of expelling the ions which build up within the collector and prevent efficient electron beam dispersal.